The present invention relates to a load cell with a foil strain gage. More specifically, the present invention relates to an electric balance with a foil strain gage capable of effectively reducing a measuring error due to an eccentric loading error.
In a conventional foil strain gage for converting a strain into a resistance change, a grid-shaped thin metal film as a resistor R is attached on an insulating base material 2 and both terminals thereof are connected to gage leads L, as shown in FIG. 4. A resistance Ro in a state without a strain, a resistance change ΔR at a rated tensile strain, and a resistance change −ΔR at a rated compressive strain are usually specified. Such a foil strain gage H is applied to load cells 1A and 1B for measuring a weight of an object as shown in FIGS. 5 and 6.
FIG. 5 is a perspective view of the load cell 1A. Foil strain gages H1 to H4 having a structure same as the foil strain gage H are bonded on strained sections S1 to S4 at an upper section 31 and a lower section 32 of a load cell body 3 formed of an elastic rectangular parallelepiped metal material having an eyeglass-shaped hollow section. One end of the load cell 1A is used as a movable column 33, and the other end thereof is used as a stationary column 34. When a load W is applied to the movable column 33, a compressive strain is generated at the strained sections S1 and S4, and a tensile strain is generated at the strained sections S2 and S3. Strains ε1 to ε4 at the strained sections S1 to S4 have magnitudes proportional to the load W and distributions in an axial direction shown in FIG. 7.
Incidentally, FIG. 7 is a graph showing a position in the axial direction and an amount of strain in case the load W is applied. An X-axis shows a location along a center line of FIG. 5, and a Y-axis shows an amount of strain.
The foil strain gages H1 to H4 are attached to the strained sections S1 to S4 as shown in FIG. 5 to form a Wheatstone bridge as shown in FIG. 8, so that the magnitudes of the strains are detected as resistance changes. Assuming that the resistance changes of the foil strain gages H1 to H4 are identical, an output voltage V is expressed by the following equation (6), wherein Ro is the resistance in a state without a strain, ΔR is the resistance change caused by the strain, and Vb is a reference voltage of the Wheatstone bridge.V=(ΔR/Ro)Vb   (6)
FIG. 6 is a perspective view of the load cell 1B. The foil strain gages H1 and H4 and the foil strain gages H2 and H3 are bonded on the strained sections S1 and S2 at the upper section 31 of the load cell body 3, respectively. With the Wheatstone bridge shown in FIG. 8, the output voltage V is obtained by the equation (6).
Patent Reference 1: Japanese Patent Publication (Kokai) No. 2003-322571
In the conventional load cells 1A and 1B, the output voltage V expressed by the equation (6) is proportional to a weight of an object, and is obtained through the Wheatstone bridge shown in FIG. 8 with the foil strain gages H1 to H4. In the case of the load cell 1A, the foil strain gages H1 to H4 are bonded over the vertical plane including the central axis of the load cell 1A, so that their center lines coincide with the central axis as shown in FIG. 5. Accordingly, there is a minimal measurement error between when the object is placed at the center and when the object is placed at an off-center position (eccentric loading errors) on a weighing pan (not shown) located above the movable column 33. However, in the load cell 1A, the foil strain gages are bonded on both the upper section 31 and the lower section 32, thereby reducing work operation efficiency.
In the case of the load cell 1B, on the other hand, the foil strain gages H1 to H4 are bonded only on the upper section 31, thereby improving work operation efficiency. However, the foil strain gages are bonded at locations away from the vertical plane including the center line of the load cell 1B, thereby inducing an eccentric loading error.
In view of the problems described above, an object of the present invention is to provide a foil strain gage capable of forming a high-precision load cell with a minimal eccentric loading error.
Further objects and advantages of the invention will be apparent from the following description of the invention.